1. Field of the Invention
This invention relates in general to infrared (IR) absorbent shielding, sometime referred to as cold shielding, and, in particular, to a method of IR shielding which utilizes macro and microscopic geometries to eliminate stray and other unwanted radiation from reaching an IR detector such as a focal plane array (FPA) to thereby improve the operation thereof
More specifically, but without restriction to the particular embodiment and/or use which is shown and described herein for purposes of illustration, this invention relates to the use of a geometric design utilizing surface reflection for selectively redirecting unwanted or stray IR radiation from the IR detector, and an IR absorbent coating on the interior of the geometric design to absorb any unwanted or stray IR radiation which passes through the geometric design or is reflected within the interior after first striking an interior surface.
2. Description of Related Art
Cold shielding is used in an IR system to allow only the desired IR radiation to be collected and focused by the optics of the system onto the IR detector or FPA. Possible sources of stray radiation which can affect the performance of the system include "off axis" objects and sources generated within the optics. Because an IR system detects and images heat, great effort must be taken to ensure that the system is protected from any source of such stray radiation "noise" that can affect its operation.
Protection against stray radiation which will cause erroneous signal detection is extremely difficult because all objects at temperatures above absolute zero emit IR radiation dependent upon their temperature and surface emissivity. Ideally, cold shields have a very low mass to aid in dynamic stability and to minimize their cool down times. In addition, an ideal cold shield must have a low emissivity exterior which will reduce the radiant parasitic heat load on the system, and will be designed such that only focused IR energy from the system optics will reach the detector. Unwanted IR energy must be reflected away from the FPA, or absorbed within the system before reaching the FPA, to insure an accurate signal detection.
One approach to improving the accuracy of IR signal detection is to use baffles placed at periodic intervals along the optical path formed between the exit pupil of the optical system and the IR detector (FPA) where the image is formed. Such an approach is described in U.S. Pat. Nos. 5,277,782 and 5,315,116. Such baffles are designed to approximate the shape of the optical "bundle" as it forms an image. Any radiation outside of the optical bundle is reflected into a cavity or re-directed out of the system by means of surface reflection which can be predicted by means of an optical ray trace. Such baffles are inserted during the manufacturing process which greatly complicates the fabrication procedure and generally results in an increase in the thermal mass of the cold shield.
The ability of a cold shield to reduce the amount of reflected energy can be improved by the utilization of a high emissivity coating which will absorb, rather than reflect, the radiation. The absorption of the system can be improved by providing a rough surface for such a high emissivity coating. One such method of roughening the surface known to those skilled in the art is referred to as "Orlando Black" or "Martin Black". Another method of roughening the surface is by creating small cavities on the surface of the mandrel used in the electroforming process for creating a cold shield by sandblasting the mandrel to roughen its surface before anodizing, as described in U.S. Pat. No. 5,196,106.
Another attempt to increase the efficiency of a cold shield is described in U.S. Pat. No. 5,298,752, which utilizes tiny retroreflectors on the interior surface of the cold shield to redirect the IR energy striking the retroreflector back in the direction from which it came.
The present invention provides a method and an apparatus for redirecting incoming light energy so that only the focused IR energy from the optical system will reach the detector or FPA, and all other radiation will be redirected from reaching the FPA through the use of a predetermined geometrical surface which reflects the undesired light energy away from the FPA, and a microrough interior surface of this geometrical shape which absorbs any of the undesired light energy which is not so reflected away.